1. Field of the Invention
The present invention pertains to seismic data processing and more particularly to data processing where converted shear waves are used in a hybrid velocity model to image reflectors located below salt wedges.
2. Related Prior Art
Seismic data is used to obtain images of subsurface formations by generating seismic waves, transmitting them into the earth to a reflector or change in density interface and detecting their arrival at a receiver. This data is in the form of time maps (e.g. seismic section, seismic trace, seismic sample, and the like) which are arranged according to time durations. The known factors are the source location, the receiver location and the time duration from generation to detection. The velocity of the seismic waves may be approximated by any one of the many methods presently in use. From these known quantities, the depth of the reflector is approximated by depth migration. Depth migration is the mapping of seismic traces on a depth scale, that is, migrating data from time to depth. This may be done either before or after mid point or common depth point (CDP) data is stacked or combined. One method for locating or identifying reflectors is by ray tracing from one point, such as the source location, to the reflector and back to a second point, such as a receiver location. Ray tracing in this manner is becoming extremely popular with the advent of high speed digital computers. However, two point ray tracing, from a source and back to a receiver, requires travel times for ray paths connecting a source position, all image points and a receiver position. This type of ray tracing requires that individual travel times be determined for each ray path from a source, to an image point and to a receiver for each image point at which the seismic wave generated at this source could be reflected and be detected at this receiver. Typically there are many hundreds of thousands of such image points. Thus, two point ray tracing is very time consuming and prohibitively expensive for computing the required travel times.
Current seismic processing images p-wave reflection energy. For salt wedges with its base dipping at greater than critical angle, the reflection from a subsalt reflector becomes a refracted wave as it enters the salt, preventing the subsalt reflector from being imaged. Converted waves enter the salt with less restriction. Therefore, prestack depth migration can be better used to analyze converted waves to form subsalt images.